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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 8 - Crops and Varieties, No. 4, Winter 1987


Insect Potential for Winter Rapeseed

Roger Veseth

The development of new alternate crops and associated management practices has been an important part of the STEEP conservation farming research program. Alternate crops not only increase economic diversity but can be very important for control of weed, disease and insect problems by lengthening crop rotation.

As a result of STEEP and related research efforts, winter rapeseed has become a viable crop option for an increasing portion of the Northwest. Higher yielding, edible and industrial varieties have been developed and production management technology has improved. Although the 1986 planted acreage is down from 1985 because of lower prices, producer interest remains high.

The Main Insect Pest

Insect damage potential is one of several rapeseed production considerations. Joe McCaffrey and Larry O'Keeffe, University of Idaho entomologists and STEEP researchers, have been involved in the development of insect control strategies for winter rapeseed. They point out that although many species of insects are commonly found on winter rapeseed, the cabbage seedpod weevil (CSPW) is by far the most important pest. CSPW larvae have been found in most cruciferous (cabbage family) crops in the Northwest, as well as tumble mustard, Jim Hill mustard and other wild relatives of the cruciferous family. Larvae of this insect feed on seeds within the seedpod and can commonly reduce rapeseed yields by 15 to 35 percent if not controlled.

Life Cycle

The CSPW produces one generation each year. It over winters as an immature adult, hibernating in protected places near the soil surface. In early spring they move to nearby cruciferous plants and eventually fly to rapeseed fields when flowering begins. As soon as seed pods are formed, the females begin laying eggs in the feeding punctures. Each female can lay 25 to 60 eggs during its lifetime. The eggs hatch in 5 to 30 days depending on the temperature and each larva can consume or destroy 5 or 6 seeds, taking 14 to 40 days to complete its development (again, depending on the temperature). Mature larvae cut a small circular hole in the pod and drop to the soil surface where they construct an earthen cell in which to pupate.

Late in the summer, new sexually immature adults emerge from the soil and feed on pods or stems that are still green. Little damage is caused by this late summer adult feeding. When rapeseed plants are fully matured, the new adult weevils fly to hibernation sites for the winter. Fig. 1 shows the typical CSPW population in relation to the crop growth stages through the growing season.

Timing and Critical Levels

Activity and development of the CSPW are very temperature dependent. Consequently, activity and development at lower elevations will usually be earlier and faster than at higher elevations. The early season egg-laying, which occurs when temperatures are cool, does not contribute significantly to yield loss. When temperatures are warmer in early June (at higher elevations), weevil activity is accelerated. Egg-laying and larval development can then result in significantly more damage, and control of the weevils at this time is most critical.

Although the population and flight activity of adult weevils peak at full bloom and continues high into early pod-set period, CSPW movement into the field declines dramatically with reduction of flowering. This is because the weevils are attracted to the crop by the yellow flower.

Economic threshold levels of adult CSPW populations have not yet been established. Preliminary results by the researchers indicate that populations as low as 3 to 6 adult CSPW per net sweep, when temperatures are warm, can cause economic loss. Because of the difficulty of trying to use a sweep net in dense rapeseed fields, McCaffrey and O 'Keeffe are researching other monitoring techniques such as yellow sticky traps. Control guidelines will be established based on insect capture, expected loss, cost of control and expected crop price.

The expected CSPW population and damage potential is influenced by many factors. If rapeseed has been grown in an area for several years or if there are many wild host plants, such as mustards, high populations might be expected. Areas that are isolated and have not had winter rapeseed or related crops or host plants will usually exhibit lower levels of damage until CSPW populations build-up.

Fig. 1. Generalized seasonal populations of adult cabbage seedpod weevils in winter rapeseed in northern Idaho.

Minor Insect Pests

Aphids

A complex of aphid species occurs on winter rapeseed. These include the cabbage aphid and turnip aphid. These insects can affect rapeseed in two ways: in the fall, feeding on rosettes can reduce plant vigor; or in the spring and summer, feeding on terminals can stunt the plants. The actual impact of aphid feeding is still unknown and research is underway. Natural predators are often important in controlling aphid populations.

Other Insects

Many other insect species can be found in winter rapeseed fields but they are generally considered minor pests. These include the flea beetles, cabbage worms, loopers, diamondback moth, lygus bugs and cabbage maggot. All, except for the cabbage maggot, have been found in winter rapeseed in northern Idaho. These insects should be monitored and evaluated as to their true pest potential. Currently no insecticides are registered for control of these pests in rapeseed.

Future Potential

The impact of a larger acreage of winter rapeseed on an increasing portion of the Northwest on the populations of these insects remains to be seen. Some increases can be expected.

Another factor that may be important to insect pest status is varietal differences in winter rapeseed. Some ''specialist feeders" such as the cabbage aphid respond to certain chemicals, such as glucosinolates, in the rapeseed plant. They use these chemicals to identify the plant as an acceptable food source. On the other hand generalist feeders such as lygus bugs, loopers and armyworms may be repelled by these same chemicals. McCaffrey and O'Keeffe point out that as the chemical components of winter rapeseed are modified with varieties for the edible and industrial markets, the insect pest species may also change.

Reference Material

Several references are available on insect management and other production aspects of winter rapeseed. Cabbage Seedpod Weevil Control in Winter Rapeseed, University of Idaho CIS 782, and Winter Rape Production Practices in Northern Idaho, University of Idaho Bulletin 634, are available from the Cooperative Extension System in Idaho. The 1986 PNW Winter Rapeseed Production Conference Proceedings, an unnumbered Pacific Northwest Extension publication, is available through the County Extension offices in Idaho, Oregon and Washington.

     
 

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